The long term objectives of this study (i) are to understand the mechanisms, at a molecular level by which spontaneous mutations which generate chromosomal aberrations occur; and (ii) to describe the genetic control of these mechanisms. The study will focus on the hemB gene of E. coli which has recently been determined to be a hot spot, at which putative insertions and large deletions have been described at the molecular level. The proposal aims to determine why the hemB gene is a hot spot by compiling an extensive catalogue of the types of chromosomal aberrations which occur there, by determining the specific nucleotide sequences which may be responsible for provoking them, (and consequently the mechanisms which may be involved), and by exploring the genetic controls of these mechanisms. The specific aims are: (1) to use Southern hybridization techniques and restriction digestion analysis to describe the kinds of aberrations which occur in hemB; (ii) to use sequencing and sequencing analysis to determine precisely the insertion sites, deletion endpoints and other sequences which may be responsible for provoking these events; (iii) to use sequencing studies to explain the nature of, and variation in, the instability, of certain mutations which map to this region of the chromosome and (iv) to use genetic techniques to describe the effects of different mutations on the frequencies of the events which occur in this region of the chromosome. A long term objective of the project is to ask whether there is a mechanistic relationship between the large deletions observed in this region of the E. coli chromosome and those associated with human diseases such as mitochondrial encephalomyopathies, beta globin variants, retinoblastoma and human porphyria.